Technical Field
The present invention relates to carrier for image forming, a developing agent, an image forming apparatus, an image forming method, replenishment toner, and a process cartridge.
Description of the Related Art
In general, in image forming methods employing electrophotography, electrostatic imaging, etc., developing agents obtained by stirring and mixing toner and carrier are used to develop latent electrostatic images formed on a latent image bearer. This developing agent is required to be a suitably-charged mixture.
To develop such latent electrostatic images, there are two known methods, one of which uses a two-component developing agent obtained by mixing toner and carrier, the other, a single-component developing agent including no carrier.
The friction charging area to toner is large due to the usage of carrier in the two-component development method. Therefore, charging properties are stable in comparison with the single-component method, which is advantageous in order to maintain the image quality for an extended period of time. In addition, the capability of supplying toner to the development area is high. For this reason, the two-component method is employed in particularly high-end apparatuses. Moreover, in digital electrophotographic system to form a latent electrostatic image on a photoconductor with a laser beam and render the latent electrostatic image visible, the two-component development method is widely adopted taking into such advantages.
The granular carrier for use in the two-component development method has been improved to prevent toner spent on the surface of the carrier, render the carrier to have a uniform surface, prevent oxidization of the surface, suppress degradation of moisture sensitivity, prolong working life of the developing agent, protect a photoconductor from being scarred or abrasion caused by the carrier, and control charging polarity and the charging size to ameliorate durability. For example, carrier covered with a particular resin material, carrier in which various additives are added to the coating resin, carrier having a surface to which an additive is attached have been proposed. In addition, a carrier coating material including a guanamine resin and a thermocuring resin cross-linkable with the guanamine resin and a carrier coating material including cross-linked matter of a melamine resin and an acrylic resin have been proposed.
Moreover, aiming for improving durability, carrier having a resin layer including a resin component in which a thermocuring resin and a guanamine resin are cross-linked and a charge control agent has been proposed. Such a resin layer is resilient enough to absorb friction with toner during stirring with toner to triboelectrically charge the developing agent or a hard shock to the coating resin caused by friction between carrier particles. For this reason, toner spent on the carrier is suppressed.
However, demand for carrier having higher level of durability is still strong on the market.
Resin coating renders the resin coated carrier insulated, which prevents the carrier from serving as a development electrode. Therefore, in particular, edge effect tends to particularly occur at solid image portion. Furthermore, since counter charge at the time of toner detachment is excessive, carrier attachment to non-imaging portion due to electrostatic development easily occurs. In an attempt to dissolve this issue, for example, resin coated carrier having a coating layer in which electroconductive carbon is dispersed as an electroconductive agent has been proposed.
However, due to friction and collision between carrier particles or carrier and toner, carbon or resin pieces including carbon are detached from such a carrier coating layer. As a result, these adhere to toner particles or are used for development meaninglessly as they are. To form a photocopying image of black texts, etc. using black toner, this phenomenon does not create a large problem. However, this arises as a significant problem such as color cloudiness or color contamination in a developing agent in combination with color toner, in particular, yellow toner, white toner, or transparent toner.
Carrier including electroconductive fillers in a carrier coating layer has been also proposed. Such carrier uses the electroconductive filler as the electroconductive particulate other than carbon. Therefore, it is possible to select less colored material to reduce an impact of colored material detached from the carrier on toner. However, the introduction of the carrier is to improve image quality by electrical stability of the electroconductive filler. Moreover, there is no mention about the color of the electroconductive filler. That is, the inclusion of the electroconductive filler is not a complete problem-solving approach to the color contamination mentioned above.
Also, carrier having an inner layer including carbon black and an outer layer including only a resin has been proposed. Since the outer layer of this carrier is coated with only a resin, contamination of toner occurs less when the coating layer is scraped.
Carrier has been proposed, which includes a resin coating layer having a concentration gradient of carbon black with a concentration from high to low from the inner side to the surface of the resin coating layer where no carbon black is present.
However, since the amount of carbon black, which serves as an only resistance control agent in such carrier, changes on the inner side and on the surface side, the carrier resistance changes as the coating layer is scraped, so that the image quality changes over time.
In addition, carrier including a layer including carbon black and a coating layer including a white-base electroconductive agent disposed on the carbon black layer have been proposed. Such carrier does not seem to significantly suffer toner contamination or a change of carrier resistance caused by scraping off of the surface of the coating layer. However, since no measures is taken to suppress scraping-off of the coating layer, the coating layer including the white electroconductive agent is easily scraped off. For this reason, if the layer including carbon black is exposed as a result of use for an extended period of time, toner contamination occurs.
Therefore, a measures to provide stable image quality for an extended period of time has been demanded while suppressing detachment from the surface of carrier and causing no color contamination even if the detachment occurs.